This invention relates to a voltage-controlled oscillator (VCO) used as a frequency standard in clock-data recovery circuitry of a high-speed serial interface, especially in a programmable logic device (PLD).
It has become common for PLDs to incorporate high-speed serial interfaces to accommodate high-speed (i.e., greater than 1 Gbps) serial input/output (I/O) standards. Many of these standards can operate at more than one clock rate. However, it is a common feature of most if not all high-speed serial data protocols that neither the clock nor information about the clock is sent with the data. Instead, the clock must be recovered from the data.
For this purpose, it is known to use “clock-data recovery” techniques in high-speed serial interfaces. Such techniques recover the clock from serial data using a closed-loop feedback system including, e.g., a phase-locked loop or delay-locked loop. Such loop circuits frequently includes VCOs, such as ring VCOs.
With high-speed serial interfaces approaching data rates of 10 Gbps, 6 GHz ring VCOs are becoming common to supply clocks for the CDR circuitry of such interfaces. In known VCOs, the gain of the VCO (KVCO) is directly proportional to the frequency—i.e., the gain increases as the frequency increases. However, VCO gain also determines the sensitivity of the output clock phase to any disturbance (or noise) on the input control voltage. As VCO gain increases to accommodate increased frequency and higher data rates, so does VCO output clock jitter due to ripples in the control voltage.
For example, VCO gain may be about 30 GHz/V at a 6 GHz oscillation frequency. This means that a 0.2 mV change in the control voltage will cause a 6 MHz frequency shift in the output. Such VCO jitter may eventually limit the input jitter tolerance of the receiver, and may even cause the CDR process to fail. This may be of particular concern in PLDs, where each particular user configuration, and therefore sensitivity to jitter, may differ.
Accordingly, it would be desirable to be able to provide a VCO with high oscillation frequency but low gain.